[fuchsia] Enable CI for branches like fuchsia_r51a. (flutter#38683)

We ended up needing a second cherrypick branch for the R51 release
as the first release candidate was abandoned. This was the convention
the Fuchsia team settled on for their branch name that we are trying to match.

Author: akbiggs

.ci.yaml

@@ -8,7 +8,7 @@
 enabled_branches:
   - main
   - flutter-\d+\.\d+-candidate\.\d+
-  - fuchsia_r\d+
+  - fuchsia_r\d+[a-z]*
 
 platform_properties:
   linux: